1. Field of the Invention
This invention relates to a substrate for a liquid crystal display device used in a display unit such as of electronic equipment and to a liquid crystal display device having the same.
2. Description of the Related Art
A liquid crystal display device, generally, has two pieces of substrates each provided with transparent electrodes, and a liquid crystal layer held between the two substrates. In the liquid crystal display device, a predetermined voltage is applied across the transparent electrodes to drive the liquid crystal, and the light transmission factor is controlled for each pixel to obtain a desired display. In recent years, the liquid crystal display device has been used for notebook PCs, TV sets, monitoring devices and projectors. The demand for the liquid crystal display devices is on the increase and is diversifying.
In the liquid crystal display device using general polarizer plates and color filters (CFs), the light transmission factor in the state of a panel is 3% to 10%, and the loss of optical energy from back light unit is great. The light transmission factor of the panel is almost determined by the light transmission factors of the polarizing plates and CF layers and by the aperture ratio of the pixels. In order to improve aperture ratio of the pixels, there has been proposed a CF-on-TFT (COT) structure in which the thin-film transistors (TFTs) and the CF layers are formed on the same substrate. In the liquid crystal display device of the COT structure, no sticking margin is necessary in sticking the TFT substrate and the opposing substrate together. Besides, the CF layer works as an interlayer insulating film being made of a thick organic material. Therefore, the ends of the pixel electrodes can be brought close to a bus line as viewing the substrate surface from a vertical direction, and the aperture ratio of the pixels can be improved.
The TFT substrate of the COT structure is fabricated through an array step and a CF step. Namely, in the COT structure, the yield of the CF step directly affects the TFT substrate. In the CF step, there are often carried out a step of forming a CF resin layer, a step of forming a resin black matrix (BM) for shutting light among the neighboring pixel regions, and a step of forming an over-coating (OC) layer on the whole surface of the CF resin layer. However, the steps of forming the resin BM and the OC layer require relatively difficult technology accounting for a drop in the production yield of the TFT substrates. In the liquid crystal display device of the COT structure, therefore, it is desired to provide neither the resin BM nor the OC layer on the TFT substrate from the standpoint of increasing the yield of production and decreasing the cost of production.
By using the CF rein layer of a large thickness of about 3.0 μm and by arranging the ends of the CF resin layer on the bus line among the neighboring pixel regions, neither the resin BM nor the OC layer may be used. That is, by forming the CF resin layer between the drain bus line and the pixel electrodes so as to work as an interlayer insulating film, there is no need of forming the OC layer. Further, by increasing the thickness of the CF resin layer, the capacitance Cds can be decreased and, hence, the ends of the pixel electrodes can be arranged close to the ends of the drain bus line and of the gate bus line (or so as to be partly overlapped) as viewing the substrate surface from a vertical direction. Namely, the two bus lines can also be used as the BM contributing to improving the aperture ratio of the pixels while omitting the BM.
FIG. 21 is a sectional view of when a conventional liquid crystal display device of the COT structure is cut in a direction perpendicular to a direction in which the drain bus line extends (JP-A-10-206888). Referring to FIG. 21, the liquid crystal display device includes a TFT substrate 102, an opposing substrate 104, and liquid crystal 106 sealed between the two substrates 102 and 104. The TFT substrate 102 has a plurality of gate bus lines (not shown) and a plurality of drain bus lines 114 intersecting the gate bus lines via an insulating film 130 on a glass substrate 110. On a pixel region on the insulating film 130, there are formed CF resin layers 140R (red), 140G (green) and 140B (blue) of three colors (FIG. 21 illustrates CF resin layers 140R and 140G) in the form of stripes extending in parallel with the drain bus line 114. On the CF resin layers 140R, 140G and 140B, a pixel electrode 116 is formed for each pixel region. The opposing electrode 104, on the other hand, has a common electrode 142 on a glass substrate 111.
An end (side end) 141R of, for example, the CF resin layer 140R is disposed on the drain bus line 114 so will not to extend to the neighboring pixel region. An end 141G of the CF resin layer 140G neighboring the CF resin layer 140R is disposed on the drain bus line 114 so will not to extend to the neighboring pixel region. The end 141G of the CF resin layer 140G is overlapping the end 141R of the CF resin layer 140R. An overlapping region where the CF resin layer 140G (end 141G) is overlapped on the CF resin layer 140R (end 141R) is formed on the drain bus line 114 only. The region where the drain bus line 114 is formed serves as a light-shielding region. Therefore, the colors are not mixed even if the CF resin layer 140G and the CF resin layer 140R are overlapped one upon the other. Further, the sum of thicknesses of the CF resin layers 140R, 140G on the overlapped region is nearly equal to the thickness of the CF resin layers 140R and 140G in other regions.
Here, the CF resin layers 140R, 140G and 140B are usually patterned relying upon a proximity exposure system having a relatively low precision. FIG. 21 illustrates an ideally patterned state. In the proximity exposure, however, a relative positional deviation may occur between the neighboring CF resin layers 140R and 140G. FIG. 22 is a sectional view illustrating the constitution of the liquid crystal display device in which the CF resin layer 140R is patterned being deviated toward the CF resin layer 140G. As illustrated in FIG. 22, the end 141R of the CF resin layer 140R extends to the neighboring pixel region exceeding the drain bus line 114. Therefore, the region where the CF resin layers 140R and 140G are overlapped extends into the pixel region G, causing the colors to be mixed and the display quality to be deteriorated.
On the other hand, if the CF resin layers 140R, 140G and 140B are patterned relying upon the step projection exposure system or the mirror projection exposure system featuring excellent positioning precision so as not to cause positional deviation of the CF resin layers 140R, 140G and 140B, the liquid crystal display device of the COT structure is produced at an increased cost.
FIG. 23 is a sectional view illustrating a conventional liquid crystal display device of the COT structure for solving the above problems (JP-A-2002-236286). In FIG. 23, the CF resin layers 140R and 140G are so formed as will not be overlapped at all or as will be overlapped only partly. Therefore, a groove portion 144 is formed on the drain bus line 114 having a groove depth nearly equal to the thickness of the CF resin layers 140R and 140G. The CF resin layers 140R and 140G do not extend to the neighboring pixel regions even if a relative positional deviation occurs during the patterning. Therefore, even when the CF resin layers 140R, 140G and 140B are formed relying upon the proximity exposure system, the quality of display is not deteriorated by the mixing of colors.
In the state illustrated in FIG. 23, however, there exists a region where the CF resin layer 140 also serving as an interlayer insulating film does not exist on the drain bus line 114 or is existing thereon but having a thickness which is not large enough. FIG. 24 illustrates the vicinity of the drain bus line 114 on an enlarged scale. Referring to FIG. 24, when the interlayer insulating film having a sufficient thickness does not exist on the drain bus line 114, the electric capacitance Cds increases between the drain bus line 114 and the pixel electrode 116 arousing a problem in that display unevenness becomes conspicuous due to crosstalk and fine deviation occurring at the seam of the exposing machine. To suppress the display unevenness, it becomes necessary to form a resin BM or to widen a gap between the drain bus line 114 and the pixel electrode 116. This causes an increase in the cost of producing the liquid crystal display device and a decrease in the light transmission factor due to a decrease in the aperture ratio of the pixels.